Thermal tuning and quadrature control using active extinction ratio tracking

ABSTRACT

Thermal tuning and quadrature control of opto-electronic devices using active extinction ratio tracking is proved by phase shifting, via a first phase shifter, a first optical signal carried on a first arm of an interferometer relative to a second optical signal carried on a second arm of the interferometer; combining the first optical signal with the second optical signal as an output signal; detecting a peak value in the output signal; and adjusting a relative phase offset imparted by the first phase shifter on the first optical signal relative to the second optical signal, based on the peak value, to increase an amplitude of the peak value. In various embodiments, the peak value is increased over time to maximize an extinction ratio of the optoelectronic device and maintain the extinction ratio in a maximized state during operation.

TECHNICAL FIELD

Embodiments presented in this disclosure generally relate to drivers foruse with Mach-Zehnder Interferometers (MZI) and other opticalmodulators. More specifically, embodiments disclosed herein activelytrack the extinction ratio at an output from an interferometer to enablethe interferometer to self-tune based on performance feedback.

BACKGROUND

In various photonic circuit elements, such as switches, modulators, andVariable Optical Attenuators (VOA), input optical signals are splitand/or combined to produce various output optical signals of desiredamplitudes. Extinction of an optical signal may occur by splitting aninput optical signal into two signals and combining the two signals tointerfere with one another so that an output optical signal has areduced amplitude from that of the input optical signal. An extinctionratio (r_(e)) can be calculated based on the relative ratio in amplitudeof the high and low output optical signals, (e.g.,r_(e)=P_(high)/P_(low)). In some photonic circuits, it is desirable toproduce an output signal such that r_(e) is below a threshold, so thatdownstream circuit elements are not inadvertently activated and so thatcarrier waves are suppressed for signal analysis. In other photoniccircuits, it is desirable to maximize r_(e) so that the input opticalsignal is used at a maximum modulation efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features of the presentdisclosure can be understood in detail, a more particular description ofthe disclosure, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate typicalembodiments and are therefore not to be considered limiting; otherequally effective embodiments are contemplated.

FIG. 1 illustrates example components of an optical signaling device,according to embodiments of the present disclosure.

FIG. 2 illustrates example components of the interferometer, accordingto embodiments of the present disclosure.

FIG. 3 illustrates a trans-impedance amplifier, according to embodimentsof the present disclosure.

FIG. 4 illustrates a central processing unit, according to embodimentsof the present disclosure.

FIG. 5 illustrates a layout of an optical signaling device on anintegrated circuit, according to embodiments of the present disclosure.

FIG. 6 illustrates a piecewise layout of an optical signaling device ontwo or more integrated circuits, according to embodiments of the presentdisclosure.

FIG. 7 is a flowchart of a method for identifying which arm of aninterferometer to select as a driven or powered arm, according toembodiments of the present disclosure.

FIG. 8 is a flowchart of a method for adjusting a relative phase offsetimparted in an interferometer, according to embodiments of the presentdisclosure.

FIG. 9 is a flowchart of a method for setting and adjusting a drivingcurrent in an interferometer to maintain the extinction ratio in asteady state, according to embodiments of the present disclosure.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially used in other embodiments withoutspecific recitation.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Overview

One embodiment presented in this disclosure provides a method forthermal tuning and quadrature control using active extinction ratiotracking that includes: phase shifting, via a first phase shifter, afirst optical signal carried on a first arm of an interferometerrelative to a second optical signal carried on a second arm of theinterferometer; combining the first optical signal with the secondoptical signal as an output signal; detecting a peak value in the outputsignal; and adjusting a relative phase offset imparted by the firstphase shifter on the first optical signal relative to the second opticalsignal, based on the peak value, to increase an amplitude of the peakvalue.

One embodiment presented in this disclosure provides a device forthermal tuning and quadrature control using active extinction ratiotracking that includes an interferometer, including: an optical input; asplitter, connected to the optical input, configured to split an opticalsignal carried by the optical input onto a first arm and a second arm,wherein the first arm comprises a first phase shifter and the second armcomprises a second phase shifter; and a combiner, connected to the firstarm and the second arm, configured to re-combine the optical signalcarried on the first arm and a second arm onto an optical output; atrans-impedance amplifier (TIA) configured to detect a peak value in theoptical output of the interferometer; and a controller, including a loopfilter, configured to adjust a phase offset between the first phaseshifter and the second phase shifter based on an amplitude of the peakvalue.

One embodiment presented in this disclosure provides a method forthermal tuning and quadrature control using active extinction ratiotracking that includes feeding a known data pattern, a random datapattern, or live data traffic to an interferometer; selecting one of afirst phase shifter disposed on a first arm of the interferometer and asecond phase shifter disposed on a second arm of the interferometer as apowered phase shifter; applying a series of phase offsets to the poweredphase shifter until a peak value at an output of the interferometerconverges to a steady state value according to a convergence threshold,wherein a phase offset associated with the steady state value isselected as an operational value; receiving an operational data streamas a first signal on the first arm and a second signal on the secondarm; phase shifting one of the first signal and the second signal by thepowered phase shifter according to the operational value; measuring thepeak value at the output of the interferometer; applying, by the poweredphase shifter, a first phase shift greater than the operational value;and in response to the peak value increasing in response applying thefirst phase shift, applying a second phase shift greater than the firstphase shift.

Example Embodiments

To provide an extinction ratio (r_(e)) that meets the thresholdrequirements of an end user, an optical device (such as aninterferometer) can be calibrated at a fabricator's facility beforebeing sent to the end user's facility and installed in a system for use.Similarly, the optical devices can be quadrature biased to produce anideal output of ((1+cos(⊖))÷2), but in practice can be tuned to providean output over a threshold or tolerance of the ideal output (e.g., arange threshold). However, heat build-up during normal operation of theoptical device can alter physical characteristics of the components ofthe optical device (e.g., due to thermal degradation or drift), thefactory calibrated set points may become unreliable in the field.Accordingly, factory test and calibration can lead to false positive andfalse negatives (passing units that fail in the field and failing unitsthat could operate in the field) or require extensive and time consumingverification of edge cases during production.

The present disclosure provides systems and methods for improvingextinction ratios in silicon photonic elements using active feedbackcontrol. Active feedback control, as discussed herein, allows for thephotonic device to adjust the phase offsets between two signals carriedby the optical device without relying on a static factory-setcalibration value (or calculated temperature-based adjustments thereto).Instead, by actively monitoring the extinction ratio signal at theoutput of the optical device, a controller for the optical device canadjust phase shifters in the optical device to ensure that the phaseoffset is properly set for the present operating conditions of theoptical device. The controller adjusts the phase offsets to ensure apeak value of the output satisfies the amplitude requirements of thedevice (e.g., an extinction ratio or amplification). Accordingly, theoptical device is continuously calibrated during operations, and allowsfor a shorter initial calibration without using a known or predefinedcalibration signal. Stated differently, live signals of (initially)unknown value are used for ensuring the optical signal strengthsatisfies a threshold value. Additionally, because calibration ismaintained during operations of the optical device, heat-build up andwear in the optical device can be accounted for, thus providing apotentially more robust and longer-lived optical device.

FIG. 1 illustrates example components of an optical signaling device100, according to embodiments of the present disclosure. The opticalsignaling device 100 may be constructed on a single chip or multiplechips to generate an optical carrier signal, impart data (viamodulation) onto the carrier signal, and transmit the data on thecarrier signal to an external device. Although various components of theoptical signaling device 100 are illustrated, one of skill in the artwill recognize that other components may also be included in the opticalsignaling device 100, such as, for example, various taps or probes tomonitor signal characteristics, a power source, amplifiers, filters,etc.

A laser 110, or other light source, generates an optical carrier signal(e.g., a continuous wave (CW) optical signal), which is output throughan interconnect 120 to an external device or transmission medium (e.g.,an optical fiber or waveguide). An interferometer 150 in the signal pathbetween the laser 110 and the interconnect 120 splits the optical signalinto two versions, which are each phase shifted and/or modulated toencode data for transmission, and then recombined for transmission. Theinterferometer 150 and components thereof are discussed in greaterdetail in regard to FIG. 2.

A serializer 130 receives the data to be encoded onto the opticalcarrier signal as electrical inputs from an external device andserializes the data according to an externally provided clock signal oran internal clock. The serializer 130 may receive the data as a packetor in another electronic format and provides the data to a first driver140 a and a second driver 140 b (generally, driver 140). The drivers 140control signal modulators (discussed in greater detail in regard to FIG.2) within an interferometer 150 to modulate the carrier signal to encodethe data thereon. The serializer 130 spaces the data transmission forencoding by the drivers 140 to account for signal propagation delaywithin the optical signaling device 100 so that the interferometer 150can encode the data reliably onto the optical carrier signal.

As illustrated, the first driver 140 a and the second driver 140 b applycomplementary signals (e.g., D₀ versus D ₀) to opposing arms within theinterferometer 150 so that the effects of the data encoding canconstructively or destructively interfere when the interferometer 150recombines the signals carried on each arm. For example, when carryingan optical carrier signal of amplitude X, each opposing arm in theinterferometer 150 nominally carries a version of the optical carriersignal of amplitude X/2, with the first arm carrying a complement of thesignal carried on the second arm (e.g., X/2 versus −X/2). In someembodiments, only one of the first driver 140 a or the second driver 140b is used at a given time; applying either D or D to the associated armof the interferometer 150 to encode a given value onto the opticalcarrier signal. In other embodiments, the first driver 140 a encodes thecomplement of the value encoded by the second driver 140 b. In someembodiments, only one driver 140 is provided, with amplifiers providingD and/or inverting amplifiers providing D to the associated arms of theinterferometer 150. When active, the drivers 140 encode the data to theoptical carrier signal via one or more modulators in an associated armof the interferometer 150 for different modulation formats (e.g., forPAM-N modulation D₀ to D_(N) will carry different data while for NRZ(Non-Return to Zero) transmission all the modulators D₀-D_(N) will carrythe same data). Additionally, in some embodiments, a one-segment driver140 can linearly drive multi-level data.

The interferometer 150 is in communication with a trans-impedanceamplifier (TIA) 160, which measure a peak value in the output of theinterferometer 150. The TIA 160 converts an electrical currentrepresenting a (tapped) optical output from the interferometer 150 intoa voltage measurement that is used by the controller 170 to controlphase shifters within the interferometer 150 to phase shift one or bothof the versions of the carrier signal on opposing arms of theinterferometer 150 before the data are encoded onto the optical signalby the drivers 140. The TIA 160 and components thereof are discussed ingreater detail in regard to FIG. 3. The controller 170 and componentsthereof are discussed in greater detail in regard to FIG. 4.

FIG. 2 illustrates example components of the interferometer 150,according to embodiments of the present disclosure. The interferometer150 includes a splitter 210, which is connected to the laser 110 orother light source, and splits optical carrier signals provided from thelaser 110 onto a first signal arm 240 a and a second signal arm 240 b(generally, signal arm 240). Each signal arm 240 carries a respectiveversion of the optical carrier that is approximately half the power ofthe original optical carrier, and applies various phase shifts andmodulations to the optical carrier to encode data before beingrecombined by the combiner 250 for transmission. The combiner 250 isconnected to the output side of each signal arm 240, and to theinterconnect 120, and is configured to re-combine the signals carried oneach of the signal arms 240 into an output signal for transmission. Thesplitter 210 and the combiner 250 can be realized as active or passiveY-junctions (amplifying or filtering one or more legs) or with otherjunction geometries in various embodiments, which may include varioustaps for measuring the characteristics of the signals carried thereon.

Each signal arm 240 includes a phase shifter 220 (a first phase shifter220 a on the first signal arm 240 a and a second phase shifter 220 b onthe second signal arm 240 b), which is a physical component that shiftsthe phase of a signal carried through that component. The embodimentsherein can be used with various types of phase shifters 220 to affectthe phase of a signal carried in a given signal arm 240, which mayoperate based on varying principals. For example, a thermo-optic phaseshifter 220 applies a controlled temperature to the transmission path ofthe signal arm 240 through which a signal is transmitted to affect aphase at which the signal exits the phase shifter 220. In someembodiments, the phase shifter 220 may be an electro-optic material,such as lithium niobate. Each phase shifter 220 ideally affects only thephase of signals passed therethrough, but in operation some losses inamplitude may be experienced, and different phase shifters 220 mayimpart different losses. By shifting the relative phases of signalscarried on parallel signal arms 240, the phase shifters 220 align therespective signals to cause destructive or constructive interference atthe combiner 250; extinguishing or amplifying the amplitude of one ormore signals.

In various embodiments, the phase shifters 220 are configured to producetwo versions of the optical carrier that are orthogonal (e.g.,quadrature biased) to one another or offset by another desired phasedifference. Stated differently, the phases of the two versions of theoptical carrier are offset by π/2 radians. In some embodiments, thefirst phase shifter 220 a may apply a phase offset of π/4 radians to theoptical carrier, while the second phase shifter 220 b may apply a phaseoffset of −π/4 radians to the optical carrier (or apply no offset) toset the two versions as orthogonal of one another. In some embodiments,the second phase shifter 220 b may be unpowered or omitted. In someembodiments, both the first phase shifter 220 a and the second phaseshifter 220 b apply offsets (e.g., +π/4 and −π/4 radians, +π/8 and −π/8radians) to the optical carrier to produce two versions of the opticalcarrier with various offsets (e.g., orthogonal). In some embodiments,the phase shifters 220 can account for phase shifts imparted bycomponents downstream from the phase shifters 220 so that the twoversions of the optical signal carried on the respective signal arms 240are orthogonal (or at another predefined relative phase) when combinedat the combiner 250.

In some embodiments, the phase shifters 220 can account for phase shiftsimparted by components downstream from the phase shifters 220 so thatthe two versions of the optical signal carried on the respective signalarms 240 are held at various desired phase offsets to one another whencombined at the combiner 250. When accounting differences inmanufacture, thermal degradation during operation, wear over a device'slifetime, etc., the phase shifters 220 may impart a phase offset that isgreater than or less than the ideal offset of π/2 radians so that thetwo versions of the carrier signal will be evenly offset at π/2 radiansat the output of the combiner 250. A driving current I_(D) (receivedfrom the controller 170 (not illustrated) or another logic device)controls one or more of the phase shifters 220 to impart the phaseoffset by actively monitoring the combined strength of the two versionsof the carrier signal at the output of the combiner 250 and adjustingthe phase offset to track the quadrature bias point of the two versionsso that the two versions are phase shifted to combine with one anotherat the desired offset.

The first signal arm 240 a and the second signal arm 240 b are alsoillustrated as including a first set of signal modulators 230 a-1through 230 a-N and a second set of signal modulators 230 b-1 through230 b-N (generally, signal modulator 230) respectively. Although eachset of signal modulators 230 is presented as including three signalmodulators 230, more or fewer than three signal modulators 230 can beincorporated in each signal arm 240 in other embodiments. Each signalmodulator 230 includes an active portion, which imparts a controlled,variable modulation to the phase of a carried signal based on a suppliedvoltage, and may optionally include an amplifying portion to offset anylosses in signal strength inherent to the signal modulator 230. Inaddition to phase modulation, a signal modulator 230 may also inducevariable amplitude modulation to boost the strength of signals carriedthrough that component.

In various embodiments, a variable portion of the signal modulator 230may be a low-doped semiconductor in asemiconductor-insulator-semiconductor-capacitor (SISCAP) arrangementthat imparts a variable drop or gain in optical signal strength. Inother embodiments, a forward-biased PIN diode or a reverse biased PNjunction device may be used in variable portion of the signal modulator230. The variable portion is controlled according to an associateddriving signal (e.g., D₀, D₁, . . . D_(N) or D ₀, D ₁, . . . D _(N))that encodes data onto the optical carrier by increasing or decreasingthe modulation order for a multi-level signaling scheme e.g., PAM-4(Pulse Amplitude Modulation, level 4), PAM-8, etc.).

FIG. 3 illustrates a TIA 160, according to embodiments of the presentdisclosure. The TIA 160 along with the controller 170 provide a logiccircuit to drive the phase shifters 220 to improve the extinction ratiobetween the two arms 240. The TIA 160 receives an electrical signalI_(FF2) based on the optical signal output from the combiner 250 of theinterferometer 150 and outputs an amplified and digitized version of theaverage value of electrical signal I_(FF2) and a digitized peak value ofthe electrical signal I_(FF2) to the controller 170 for analysis.

The TIA 160, at a first node 301, receives the combined signal outputfrom the combiner 250 of the interferometer 150 as an input. Aninverting amplifier 310 and a resistor 320 are connected in parallelbetween the first node 301 and a second node 302 to translate theI_(FF2) current into a voltage and amplify the input combined signal.

A first current source 330 a (generally, current source 330), such as aMetal Oxide Field Effect Transistor (MOSFET) or other poweredsemiconductor device, is disposed between the first node 301 and ground340, and is controlled via a direct current (DC) feedback line 350connected to a gate of the first current source 330 a to conduct the DCoffset portion of the measured combined signal to ground.

Additionally, the DC feedback line 350 is connected to a gate of asecond current source 330 b, which is connected between ground 340 andan input for a first Analog to Digital Converter (ADC) 360 a (generally,ADC 360) to provide a digitized version of the electrical signal I_(FF2)to the controller 170.

An input of the programmable gain amplifier 370 is connected to thesecond node 302, and produces both an amplified version and anamplified-inverted version of the electrical signal I_(FF2). A peakdetector 380 receives the amplified and amplified-inverted version toidentify a peak value in the electrical signal I_(FF2). The peak valueis digitized by a second ADC 360 b, and output to the controller 170.

FIG. 4 illustrates a controller 170, according to embodiments of thepresent disclosure. The controller 170 may be implemented in amicroprocessor or other logic control device that includesinitialization logic 410, a digital loop filter 420, and a selector 430and a digital to analog converter (DAC) 440.

The initialization logic 410 determines which arm 240 of theinterferometer 150 is to be driven by the driving current I_(D) and whatthe initial amplitude of the driving current I_(D) is to be. In variousembodiments, the initialization logic 410 is performed at devicestart-up, but may also be performed periodically (e.g., every sseconds), in response to a reset command, or at other times indicated byan operator. The initialization logic 410 monitors the extinction ratioof the interferometer 150 via the output I_(FF2) at several known valuesto determine the slope between an un-phase shifted signal and signalsphase shifted with a known amount of power on one of each of the arms240. In some embodiments, the initialization logic 410 can perform someor all of the operations described in greater detail in regard to FIG.7.

The digital loop filter 420 receives the peak value determined by thepeak detector 380 as an input and determines how to adjust the drivingcurrent I_(D) to ensure that the extinction ratio for the interferometer150 is maintained or improved upon over time. As the adjustmentsincrease or decrease the extinction ratio, the digital loop filter 420alters the amount of power used to drive a phase shifter 220 to alterthe phase offset imparted to a corresponding version of the carriersignal on that signal arm 240. In some embodiments, the digital loopfilter 420 can perform some of all of the operations described ingreater detail in regard to FIG. 8.

The selector 430 receives inputs from the initialization logic 410 andfrom the digital loop filter 420, and determines which one's output toprovide as the driving current I_(D) to one or more of the phaseshifters 220. Additionally, based on input from the initialization logic410, the selector 430 can select which phase shifter 220 receives whichdriving current I_(D) (e.g., the first/second phase shifter 220 a/220 breceives the driving current I_(D)).

FIG. 5 illustrates a layout of an optical signaling device 100 on anintegrated circuit 500, according to embodiments of the presentdisclosure. The integrated circuit 500 can include some or all of theelements of the optical signaling device 100 described in FIG. 1, theinterferometer 150 described in FIG. 2, the trans-impedance amplifier160 described in FIG. 3, and the controller 170 described in FIG. 4, aswell as additional components. For example, the laser 110 may beincluded in the integrated circuit 500 or be fabricated on a separatechip from the other components of the optical signaling device 100 canbe linked to the interferometer 150 via a laser interconnect 510.

In embodiments using a laser interconnect 510, the laser interconnect510 includes a fastener 511 to hold the laser 110 in place. Thewaveguides of the laser 110 are aligned at an input optical coupling 512relative to the waveguides of the interferometer 150 used to receivelight. The input optical coupling 512 may include various opticalcoatings and surface treatments to reduce back reflection and otheraberrant optical signaling conditions, and may include butt-coupledjoints and evanescent joints to receive the optical carrier signal fromthe laser 110 and transmit that optical carrier signal to theinterferometer 150. Additionally, an input tap 513 can be included tomeasure a predefined percentage of the strength of the optical carriersignal carried thereover (e.g., 5%), the output of which may be providedas an electrical output (I_(FF1)) for use by a controller 170 (e.g., onan external logic circuit) by an input optical converter 514 (e.g., aphotodiode to convert optical signals to electrical signals).

In some embodiments the interconnect 120 includes similar elements asthe laser interconnect 510, but on the output side of the interferometer150. Accordingly, the interconnect 120 can provide a logic circuit withdetails on the signal characteristics of the output data-encoded signalfrom the interferometer 150 and ensure a proper connection with anexternal device or optical cable for transmission. An output tap 523 canbe included to measure a predefined percentage of the amplitude of thedata-encoded optical carrier signal carried thereover (e.g., 5%), theoutput of which is provided as an electrical output (I_(FF2)) for use bythe TIA 160 and controller 170 by an output optical converter 524 (e.g.,a photodiode to convert optical signals to electrical signals). Anoutput fastener 521 is included to hold the output waveguides ofinterferometer 150 in alignment at an output optical coupling 522 withthe transmission cables or external device receiving the data-encodedsignal. The output optical coupling 522 may include various opticalcoatings and surface treatments to reduce back reflection and otheraberrant optical signaling conditions, and may include butt-coupledjoints and evanescent joints to receive the data-encoded signal from theinterferometer 150 and transmit the data externally from the integratedcircuit 500.

FIG. 6 illustrates a piecewise layout 600 of an optical signaling device100 on two or more integrated circuits, according to embodiments of thepresent disclosure. As will be appreciated, different components of anoptical signaling device 100 may be fabricated according to differentprocesses, and fabricators may produce various components as modularentities that can be swapped in or out of different optical signalingdevices 100 (or other circuits) as engineering requirements dictate.Additionally, the physical structures of one component may beincompatible with the construction processes of another component.Therefore, a fabricator may desire to layout the optical signalingdevice in one or more a photonic integrated circuits (PICs) 610 thatinclude the optical components of the optical signaling device 100, andin one or more electrical integrated circuit (EIC) 620 that include theelectrical components of the optical signaling device 100.

The PIC 610 and the EIC 620 may be fabricated on separate wafers withseveral dies defining several instances of the respective PICs 610 orEIC 620 that use different substrates, deposition or growth processes,and/or form different sub-components made of different materials. ThePICs 610 and EICs 620 can be combined into the optical signaling device100 by various wire bonds, interposer circuits, or direct connectionsbetween the associated electrical inputs and outputs of the PICs 610 andEIC 620. For example, a wire may be connected to an output of a driver140 and to the input of a signal modulator 230 to allow the driver 140to electrically control the signal modulator 230. In another example,the output optical converter 524 may be connected to a trace on aninterposer circuit that is also connected to the input of the TIA 160 tosupply the TIA 160 with an electrical signal corresponding to thestrength of the optical carrier. The PIC and EIC can be manufactured ona single wafer as well.

FIG. 7 is a flowchart of a method 700 for identifying which arm 240 ofan interferometer 150 to select to have the powered phase shifter 220,according to embodiments of the present disclosure. Method 700 beginswhen the optical signaling device 100 is first powered on, in responseto a reset command, or periodically per device settings.

At block 710, the optical signaling device 100 measures the outputsignal from the interferometer 150 while applying a series of fixedpowers to the phase shifters 220 on each of the signal arms 240. Theinitialization logic 410 identifies an initial output current I_(FF2-0)when a signal is passed through the interferometer 150 without a phaseadjustment to either of the phase shifters 220 (i.e., the fixed powerapplied to both the first phase shifter 220 a and the second phaseshifter 220 b is zero Watts). The initialization logic 410 then appliesa known power to the first phase shifter 220 a (i.e., a fixed power of XWatts is applied to the first phase shifter 220 a and zero Watts to thesecond phase shifter 220 b), and identifies a first offset outputcurrent I_(FF2-1). Similarly, the initialization logic 410 applies theknown power to the second phase shifter 220 b (i.e., a fixed power of XWatts is applied to the second phase shifter 220 b and zero Watts to thefirst phase shifter 220 a), and identifies a second offset outputcurrent I_(FF2-2).

At block 720, the initialization logic 410 determines whether slopepolarity is the same in both arms 240 or is different. The initialoutput current I_(FF2-0) is used relative to the first offset outputcurrent I_(FF2-1) to determine the slope for the first arm 240 aaccording to Formula 1. The initial output current I_(FF2-0) is usedrelative to the second offset output current I_(FF2-2) to determine theslope for the second arm 240 b according to Formula 2.Slope₁ =I _(FF2-1) −I _(FF2-0)  [Formula 1]Slope₂ =I _(FF2-0) −I _(FF2-2)  [Formula 2]

If the slope for the first arm 240 a and the slope for the second arm240 b are different polarities from one another (e.g., one is negativeand the other is positive), the quadrature bias points are equidistantfrom the initial un-phase shifted measurement I_(FF2-0), and eitherphase shifter 220 may be selected to phase shift the optical carrier(because both use an equivalent amount of power to reach the quadraturepoint). Method 700 then proceeds to block 730, where either arm 240 isselected as the powered/driven arm 240. In various embodiments, at block730, the initialization logic 410 selects one of the arms 240 based onthat arm 240 being identified as a default or preferred arm 240, basedon a random number generator or any other selection method for selectingone of the arms 240, etc. Method 700 may then conclude.

If the slope for the first arm 240 a and the slope for the second arm240 b are the same polarity as one another (e.g., both are positive orboth are negative), the shared polarity indicates that one of the firstarm 240 a and the second arm 240 b is closer to the quadrature biaspoint than the other. Accordingly, a lower amount of power can beapplied to one of the corresponding phase shifters 220 to phase shiftthe carrier signal to reach the phase shift needed to achieve bias therelative offset of the two versions of the carrier signal to thequadrature point than is needed to be applied to the other phase shifter220.

At block 740, when the first slope₁ and the second slope₂ are bothpositive, method 700 proceeds to block 750. At block 740, when the firstslope₁ and the second slope₂ are both negative, method 700 proceeds toblock 760.

At block 750, the initialization logic 410 selects the phase shifter 220in one of the first arm 240 a and the second arm 240 b to be the activeor powered phase shifter 220 to which the driving current I_(D) is to beapplied. When the arm slopes are both positive, the interferometer 150selects the second arm 240 b as the second arm 240 b is closer to thequadrature point, and will take less power to achieve the quadraturebias. For example, when both slope₁ and slope₂ are positive, the drivingcurrent I_(D) supplied to the second phase shifter 220 b to reach themaximum extinction ratio is less than the driving current I_(D) suppliedto the first phase shifter 220 a to reach the maximum extinction ratio,and the initialization logic 410 therefore selects the second arm 240 b.Method 700 may then conclude.

At block 760, the initialization logic 410 selects the phase shifter 220in one of the first arm 240 a and the second arm 240 b to be the activeor powered phase shifter 220 to which the driving current I_(D) is to beapplied. When the arm slopes are both negative, the interferometer 150selects the first arm 240 a as the first arm 240 a is closer to thequadrature point, and will take less power to achieve the quadraturebias. For example, when slopes and slope₂ are negative, the drivingcurrent I_(D) supplied to the first phase shifter 220 a to reach themaximum extinction ratio is less than the driving current I_(D) suppliedto the second phase shifter 220 b to reach the maximum extinction ratio,and the initialization logic 410 therefore selects the first arm 240 a.Method 700 may then conclude.

In embodiments in which method 700 is repeated, the initialization logic410 may select different phase shifters 220 at different times. Forexample, if the initialization logic 410 selects the first phase shifter220 a to be the powered phase shifter 220 when the optical signalingdevice 100 is first activated, but is powered down at a later time, whenthe optical signaling device 100 is next activated, the initializationlogic 410 may select either the first phase shifter 220 a or the secondphase shifter 220 b to be the powered phase shifter 220. Accordingly, asthe efficiency of one phase shifter 220 changes relative to the otherphase shifter 220 over time (e.g., due to thermal degradation, wear,etc.), the optical signaling device 100 can be recalibrated in the fieldto select the most power efficient phase shifter 220 to use as thepowered phase shifter 220.

FIG. 8 is a flowchart of a method 800 for adjusting a relative phaseoffset imparted in an interferometer 150, based on the peak value ofsignals output from the interferometer 150, to increase an amplitude ofthe peak value and thereby improve the biasing of the interferometer150, according to embodiments of the present disclosure.

Method 800 begins with block 810, where a splitter 210 in theinterferometer 150 receives an optical signal for use as a calibrationsignal and/or data transmission signal, and splits that optical signalonto a first signal arm 240 a and a second signal arm 240 b. At block820, a phase shifter 220 on one of the signal arms 240 (e.g., asselected per method 700 described in greater detail in regard to FIG. 7)phase shifts the version of the optical signal carried on thecorresponding signal arm 240. At block 830, a combiner 250 in theinterferometer 150 receives the version of the optical signal carried oneach of the signal arms 240 (which may be modulated to encode datathereon) and recombines the phase shifted signal with the un-phaseshifted signal to produce an output signal. Blocks 810-840 are performedcontinuously so long as an optical signal is fed to the interferometer150.

At block 840, a portion of the output signal (e.g., 5% captured by atap) is converted to an electrical signal I_(FF2), from which a TIA 160detects the peak value in the electrical signal I_(FF2).

In a first iteration of method 800, method 800 proceeds from block 840to block 845. Otherwise, in subsequent iterations of method 800, method800 proceeds from block 840 to block 850. For example, method 800 maydetect an initial peak value in the output signal and set an initialphase offset during an initial iteration, and in a subsequent iterationdetects a subsequent peak value in the output signal, and adjusts therelative phase offsets accordingly to maintain the peak value in asteady state to provide a maximized extinction ratio as time progresses.

At block 845, the controller 170 selects the arm 240 in which phaseshift needs to be applied based on the output of method 700 and adjuststhe phase offset in a first direction (e.g., by increasing or decreasingthe power applied to the powered phase shifter 220), and the TIA 160identifies a new peak value from the resulting electrical signalI_(FF2).

At block 850, once the peak value measurements have begun, thecontroller 170 monitors the peak value to add to or remove from thephase shift to increase the extinction ratio until the maximumextinction ratio is reached, and then maintains the phase shift in asteady state for the maximized extinction ratio. In various embodiments,the controller 170 determines whether the peak value of the mostrecently measured output signal I_(FF2) is higher or lower than the peakvalue of the previously measured output signal I_(FF2) and accordinglyhow the driving current I_(D) should be adjusted to increase the nextpeak value. For example, the controller 170 compares the peak value attime t₀ to the peak value at time t₁, the peak value at time t₁ to thepeak value at time t₂, etc.

When the current peak value is greater than the previous peak value,method 800 proceeds from block 850 to block 860, where the controller170 adjusts the phase offset in the same direction as in the previousiteration. For example, when the controller 170 adjusted the powerupward from time t₀ to time t₁, at block 860 the controller 170 adjuststhe power upward again for time t₂ (e.g., P₀<P₁<P₂). In another example,when the controller 170 adjusted the power downward from time t₇ to timet₈, at block 860 the controller 170 adjusts the power downward again fortime t₉ (e.g., P₇>P₈>P₉). Method 800 returns from block 860 to block840, where method 800 performs a next iteration with the newly adjusteddriving current I_(D).

When the current peak value is less than the previous peak value, method800 proceeds from block 850 to block 870, where the controller 170adjusts the phase offset in the opposite direction from the previousiteration. For example, when the controller 170 adjusted the powerupward from time t₀ to time t₁, at block 870 the controller 170 adjuststhe power downward for time t₂ (e.g., P₀<P₁, P₁>P₂). In another example,when the controller 170 adjusted the power downward from time t₇ to timet₈, at block 870 the controller 170 adjusts the power upward for time t₉(e.g., P₇>P₈, P₈<P₉). Method 800 returns from block 870 to block 840,where method 800 performs a next iteration with the newly adjusteddriving current I_(D).

In consecutive iterations through block 840 of method 800, thecontroller 170 may adjust the power supplied to the phase shifter 220upward-upward, downward-downward, upward-downward, or downward-upward.When the power supplied to the phase shifter 220 alternates betweenbeing increased and decreased in subsequent iterations (e.g., theupward-downward or downward-upward patterns), the phase shifter 220 isin a steady state that corresponds to a maximized value for theextinction ratio, which the controller 170 seeks to maintain within arange threshold for the output of the interferometer 150 to therebystabilize the peak value to the maximized value.

FIG. 9 is a flowchart of a method 900 for setting and adjusting adriving current in an interferometer 150 to maintain the extinctionratio in a steady state (within a range threshold), according toembodiments of the present disclosure.

At block 910, the optical signaling device 100 receives a known datapattern from the serializer 130, which is fed to the interferometer 150.In some embodiments, block 910 may be omitted and method 900 isperformed with an operational signal to encode data of values that arenot known a priori.

At block 920, the optical signaling device 100 performs method 700 (asdescribed in relation to FIG. 7) as a calibration phase to identify andselect one phase shifter 220 to use as a powered or active phase shifter220 in the interferometer 150 (and for the other phase shifter 220 to beun-powered). The selected phase shifter 220 represents the phase shifter220 that uses the least amount power of the two phase shifters 220 toimpart a phase shift that biases the optical carrier signal to thequadrature point when recombined at the combiner 250 of theinterferometer 150.

At block 930, the optical signaling device 100 performs method 800 untilthe driving current I_(D) converges on the steady state value for theinterferometer 150, at which point method 900 proceeds to block 940,where the steady state value for the driving current I_(D) is frozen inthe digital loop filter 420, and the known data pattern is stopped.Convergence to the steady state value can occur when the opticalsignaling device 100 identifies that the output is at a maximum value orwithin a convergence threshold, such as when the driving current I_(D)swaps between increasing and decreasing in subsequent rounds or thechange in the output is less than a preset amount.

At block 950, the interferometer 150 receives operational traffic (i.e.,a live optical signal with initially unknown data to be modulatedthereon), and proceeds to block 960.

At block 960, the optical signaling device 100 unfreezes the digitalloop filter 420 to use the steady state value for the driving currentI_(D) as an initial value when performing method 800 for a second time.The second performance of method 800 may continue until the opticalsignaling device 100 is shut down, reset, or the input signal isinterrupted, at which time method 900 may also conclude or restart.

In the current disclosure, reference is made to various embodiments.However, the scope of the present disclosure is not limited to specificdescribed embodiments. Instead, any combination of the describedfeatures and elements, whether related to different embodiments or not,is contemplated to implement and practice contemplated embodiments.Additionally, when elements of the embodiments are described in the formof “at least one of A and B,” it will be understood that embodimentsincluding element A exclusively, including element B exclusively, andincluding element A and B are each contemplated. Furthermore, althoughsome embodiments disclosed herein may achieve advantages over otherpossible solutions or over the prior art, whether or not a particularadvantage is achieved by a given embodiment is not limiting of the scopeof the present disclosure. Thus, the aspects, features, embodiments andadvantages disclosed herein are merely illustrative and are notconsidered elements or limitations of the appended claims except whereexplicitly recited in a claim(s). Likewise, reference to “the invention”shall not be construed as a generalization of any inventive subjectmatter disclosed herein and shall not be considered to be an element orlimitation of the appended claims except where explicitly recited in aclaim(s).

As will be appreciated by one skilled in the art, the embodimentsdisclosed herein may be embodied as a system, method or computer programproduct. Accordingly, embodiments may take the form of an entirelyhardware embodiment, an entirely software embodiment (includingfirmware, resident software, micro-code, etc.) or an embodimentcombining software and hardware aspects that may all generally bereferred to herein as a “circuit,” “module” or “system.” Furthermore,embodiments may take the form of a computer program product embodied inone or more computer readable medium(s) having computer readable programcode embodied thereon.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF (Radio Frequency), etc., or anysuitable combination of the foregoing.

Computer program code for carrying out operations for embodiments of thepresent disclosure may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Smalltalk, C++ or the like and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

Aspects of the present disclosure are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatuses(systems), and computer program products according to embodimentspresented in this disclosure. It will be understood that each block ofthe flowchart illustrations and/or block diagrams, and combinations ofblocks in the flowchart illustrations and/or block diagrams, can beimplemented by computer program instructions. These computer programinstructions may be provided to a processor of a general purposecomputer, special purpose computer, or other programmable dataprocessing apparatus to produce a machine, such that the instructions,which execute via the processor of the computer or other programmabledata processing apparatus, create means for implementing thefunctions/acts specified in the block(s) of the flowchart illustrationsand/or block diagrams.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other device to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the block(s) of the flowchartillustrations and/or block diagrams.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other device to cause aseries of operational steps to be performed on the computer, otherprogrammable apparatus or other device to produce a computer implementedprocess such that the instructions which execute on the computer, otherprogrammable data processing apparatus, or other device provideprocesses for implementing the functions/acts specified in the block(s)of the flowchart illustrations and/or block diagrams.

The flowchart illustrations and block diagrams in the Figures illustratethe architecture, functionality, and operation of possibleimplementations of systems, methods, and computer program productsaccording to various embodiments. In this regard, each block in theflowchart illustrations or block diagrams may represent a module,segment, or portion of code, which comprises one or more executableinstructions for implementing the specified logical function(s). Itshould also be noted that, in some alternative implementations, thefunctions noted in the block may occur out of the order noted in theFigures. For example, two blocks shown in succession may, in fact, beexecuted substantially concurrently, or the blocks may sometimes beexecuted in the reverse order, depending upon the functionalityinvolved. It will also be noted that each block of the block diagramsand/or flowchart illustrations, and combinations of blocks in the blockdiagrams and/or flowchart illustrations, can be implemented by specialpurpose hardware-based systems that perform the specified functions oracts, or combinations of special purpose hardware and computerinstructions.

In view of the foregoing, the scope of the present disclosure isdetermined by the claims that follow.

We claim:
 1. A method comprising: phase shifting, via a first phaseshifter, a first optical signal carried on a first arm of aninterferometer relative to a second optical signal carried on a secondarm of the interferometer, wherein the first optical signal is acalibration signal generated on an optical device including theinterferometer to initialize the first phase shifter; combining thefirst optical signal with the second optical signal as an output signal;detecting a peak value in the output signal; adjusting a relative phaseoffset imparted by the first phase shifter on the first optical signalrelative to the second optical signal, based on the peak value, toincrease an amplitude of the peak value; phase shifting, via the firstphase shifter according to the relative phase offset, the first opticalsignal carried on the first arm relative to the second optical signalcarried on the second arm; combining the first optical signal with thesecond optical signal as the output signal; detecting a second peakvalue in the output signal; adjusting the relative offset imparted bythe first phase shifter on the first optical signal relative to thesecond optical signal, based on the second peak value until the secondpeak value converges within a range threshold to a steady state value;and setting the relative offset associated with the steady state valueas an operational offset for the first phase shifter.
 2. The method ofclaim 1, wherein the first optical signal is an operational signalgenerated to carry data to an external device, wherein the data areencoded onto the operational signal on at least one of the first arm andthe second arm.
 3. The method of claim 1, wherein detecting the peakvalue and adjusting the relative phase offset are performed continuouslyduring operations of the interferometer.
 4. The method of claim 1,further comprising: detecting a subsequent peak value in the outputsignal, wherein adjusting the relative phase offset comprises one ofincreasing the relative phase offset and decreasing the relative phaseoffset; and in response to the subsequent peak value being less than thepeak value, adjusting the relative phase offset imparted by the firstphase shifter on the first optical signal relative to the second opticalsignal according to the one of increasing the relative phase offset anddecreasing the relative phase offset.
 5. The method of claim 1, furthercomprising: detecting a subsequent peak value in the output signal,wherein adjusting the relative phase offset comprises one of increasingthe relative phase offset and decreasing the relative phase offset; andin response to the subsequent peak value being less than the peak value,adjusting the relative phase offset imparted by the first phase shifteron the first optical signal relative to the second optical signalaccording to a different one of increasing the relative phase offset anddecreasing the relative phase offset.
 6. The method of claim 1, whereinthe first phase shifter is selected as a powered phase shifter, and asecond phase shifter included on the second arm is selected as anun-powered phase shifter based on a calibration phase performed at poweron of the interferometer indicating that the first phase shifterrequires less power to bias the first optical signal to a quadraturepoint than the second phase shifter.
 7. A device, comprising: aninterferometer, including: an optical input; a splitter, connected tothe optical input, configured to split an optical signal carried by theoptical input onto a first arm and a second arm, wherein the first armcomprises a first phase shifter and the second arm comprises a secondphase shifter; and a combiner, connected to the first arm and the secondarm, configured to re-combine the optical signal carried on the firstarm and a second arm onto an optical output; a trans-impedance amplifier(TIA) configured to detect a peak value in the optical output of theinterferometer; and a controller, including a loop filter, configured toadjust a phase offset between the first phase shifter and the secondphase shifter based on an amplitude of the peak value; and wherein thefirst phase shifter is selected as a powered phase shifter and thesecond phase shifter is selected as an un-powered phase shifter based ona calibration phase performed at power on of the device indicating thatthe first phase shifter requires less power to bias the optical signalto a quadrature point than the second phase shifter.
 8. The device ofclaim 7, wherein the optical signal used during the calibration phase isan operational signal of an initially unknown value carrying livetraffic data.
 9. The device of claim 7, wherein the optical signal usedduring the calibration phase is a known data pattern signal.
 10. Thedevice of claim 9, wherein the calibration phase is performed until aphase offset converges on a steady state value, wherein the first phaseshifter is frozen to the phase offset until an operational signal of aninitially unknown value is transmitted through the device.
 11. Thedevice of claim 7, wherein the loop filter adjusts the phase offsetbetween the first phase shifter and the second phase shifter based onthe amplitude of the peak value to stabilize the peak value at amaximized value.
 12. The device of claim 11, wherein the maximized valuechanges over time based on thermal degradation of the interferometer.13. A method, comprising: feeding a known data pattern to aninterferometer; selecting one of a first phase shifter disposed on afirst arm of the interferometer and a second phase shifter disposed on asecond arm of the interferometer as a powered phase shifter; applying aseries of phase offsets to the powered phase shifter until a peak valueat an output of the interferometer converges to a steady state valueaccording to a convergence threshold, wherein a phase offset associatedwith the steady state value is selected as an operational value;receiving an operational data stream as a first signal on the first armand a second signal on the second arm; phase shifting one of the firstsignal and the second signal by the powered phase shifter according tothe operational value; measuring the peak value at the output of theinterferometer; applying, by the powered phase shifter, a first phaseshift greater than the operational value; and in response to the peakvalue increasing in response applying the first phase shift, applying asecond phase shift greater than the first phase shift.
 14. The method ofclaim 13, wherein the peak value is detected via a trans-impedanceamplifier.
 15. The method of claim 13, wherein selecting the one of thefirst phase shifter and the second phase shifter as the powered phaseshifter further comprises: measuring an initial peak value withoutpowering the first phase shifter and the second phase shifter; measuringa first peak value when powering the first phase shifter with a fixedamount of power and not powering the second phase shifter; measuring asecond peak value when powering the second phase shifter with the fixedamount of power and not powering the first phase shifter; determining afirst slope based on the initial peak value and the first peak value;determining a second slope based on the initial peak value and thesecond peak value; and selecting, based on the first slope and thesecond slope, the one of the first phase shifter and the second phaseshifter that requires less power to bias an optical signal to aquadrature point.
 16. The method of claim 15, in response toidentifying, based on the first slope and the second slope, that thefirst phase shifter and the second phase shifter require equivalentpower to bias the optical signal to the quadrature point, selecting thefirst phase shifter as the powered phase shifter.
 17. The method ofclaim 13, further comprising: measuring the peak value at the output ofthe interferometer at a time after applying the second phase shift; andin response to the peak value decreasing in response applying the secondphase shift, applying a third phase shift less than the second phaseshift.
 18. The method of claim 13, further comprising: measuring thepeak value at the output of the interferometer at a time after applyingthe second phase shift; and in response to the peak value increasing inresponse applying the second phase shift, applying a third phase shiftgreater than the second phase shift.